1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer, direct digital master making apparatus or similar electrophotographic image forming apparatus. More particularly, the present invention relates to a developing device included in the image forming apparatus and using a magnetic force, a magnet roller for use in a developing roller included in the developing device, a magnet molding that forms part of the developing roller, and a magnet compound material for producing the magnet molding.
2. Description of the Background Art
It is a common practice with an electrophotographic image forming apparatus to form a latent image on a photoconductive drum, belt or similar image carrier in accordance with image data and then develop the latent image with a developing device to thereby produce a corresponding toner image. Development for such an electrophotographic system is, in many cases, uses a magnet brush. More specifically, when use is made of a two-component type developer made up of toner and magnetic grains, the developer is magnetically deposited on the surface of the image carrier, forming a magnet brush. In a developing zone where an electric field for development is formed between the image carrier and a developer carrier, the toner is selectively transferred from the magnet brush to the latent image on the image carrier by an electric field formed between the image carrier and a sleeve applied with an electric bias.
A developing device configured to satisfy both of developing conditions for increasing image density and those for forming a desirable low-contrast image (SLIC developing device hereinafter) is disclosed in, e.g., Japanese Patent Laid-Open Publication No. 2000-305360. The SLIC developing device, capable of solving problems relating to images formed by the two-component type developer, uses a developing roller characterized in that a main pole for development has a half-value width of 22° or 20°, as the case may be, and flux density of 100 mT to 130 mT. A half-value width refers to an angular width between positions where the magnetic force is one-half of the maximum magnetic force or peak of a magnetic force distribution curve in the normal direction, and has conventionally been about 50° in the case of the two-component type developer. Flux density in the case of the two-component type developer has conventionally been between 80 mT and 120 mT.
As stated above, the main pole of the SLIC developing device needs high flux density and a half-value width that is one-half of conventional one or less. As for a conventional ferrite magnet roller, a decrease in half-value width directly translates into a decrease in flux density, preventing required performance from being attained. It is therefore necessary to use a material having a high energy product. While the specifications of the developing roller are dependent on the type of an image forming apparatus and roller diameter, flux density of 100 mT to 130 mT is required of the main pole and poles adjoining it in recent image forming apparatuses, increasing the demand for a higher magnetic force. Translating flux density on a developing roller into a (BH)max value representative of the magnetic force of a magnet, 100 mT to 130 mT corresponds to 13 mGOe to 16 mGOe. Therefore, a magnet whose magnetic force is 13 mGOe or above is essential.
While Sm—Co, Nd—Fe—B and Sm—Fe—N rare earth magnets are known in the art as magnet materials having high energy products, today Nd—Fe—B and Sm—Fe—N are predominant over Sm—Co because Sm—Co is expensive. To provide the magnet with a desired configuration, it is necessary to use a so-called plastic magnet or resin magnet formed by kneading plastic resin.
Generally, a plastic magnet is produced by any one of injection molding, extrusion molding, and compression molding. These molding schemes each have merits and demerits, as will be described hereinafter. Injection molding can implement accurate molding because dimensions are determined by a mold. However, to allow a magnet material to flow through a mold with high fluidity, it is necessary to increase the blending ratio of resin while limiting the blending ratio of a magnet, preventing a magnet from achieving a strong magnetic force. Extrusion molding, which effects continuous molding, enhances productivity, but is lower in dimensional accuracy than injection molding. Further, extrusion molding, like injection molding, limits the blending ratio of a magnet and therefore a magnetic force. Compression molding increases density by pressing a magnet material and is desirable for providing a magnet with a strong magnetic force. However, compression molding is applicable only to small parts because it cannot produce large magnets without resorting to a large-scale press.
Further, the conventional molding schemes stated above use thermosetting resin without exception. Consequently, the resulting magnets can be stored only for an extremely short period of time and therefore cannot be stabilized in quality as products. In light of this, Japanese Patent Laid-Open Publication No. 4-11702, for example, discloses a method of producing a plastic magnet by mixing fine powder of resin and magnetic powder, molding the resulting powdery mixture by compression while applying or not applying a magnetic field, and heating the resulting molding. While this method is a kind of compression molding, the above resin powder is thermoplastic resin or so-called B-stage thermosetting resin. The magnetic powder is open to choice and may be anyone of, e.g., ferrite powder, rare earth-cobalt powder, alnico powder, and neodymium-iron-boron powder.
Generally, an anisotropic magnet material implements a stronger magnetic force than an isotropic magnet material. In the event of molding, an anisotropic magnet material is subject to a magnetic field for orientation in order to achieve a strong magnetic force. Today, an Nd—Fe—B material, which is provided with high anisotropy by high-temperature hydrogen processing, is available as a rare earth material with a strong magnetic force, as taught in, e.g., Japanese Patent Laid-Open Publication Nos. 10-13517 and 8-31677.
Although plastic, rare earth magnet molding produced by the injection molding or the protrusion molding of isotropic Nd—Fe—B is available on the market, the magnetic force of such a magnet molding is only 6 mGOe to 9 mGOe in terms of (BH)max. To provide a magnet for the SLIC developing device with a magnetic force of 13 mGOe or above, we studied the use of an anisotropic Nd magnet having the strongest magnetic force available today. However, when injection molding or protrusion molding was used, even the anisotropic Nd magnet exhibited a magnetic force of only 10 mGOe to 12 mGOe short of 13 mGOe.
We therefore conducted a series of extended researches and experiments for finding a compression molding method implementing the strongest magnetic force. An anisotropic material must be subject to a magnetic field during molding. Apart from the teachings of Laid-Open Publication No. 4-11702 mentioned earlier, a compound for compression molding is usually implemented by an epoxy material, which is thermosetting resin. The epoxy resin and a hardener are blended by 1 wt. % to 10 wt. % and deposited on magnet powder to thereby constitute a dry compound. However, to make the epoxy resin a dry compound, it is necessary to use solid epoxy resin and a solid harder. While many different materials, including aromatic amine, dicyan-diamide and imidazole, are available for a solid hardener, such materials all have high setting points and need at least 150° C. In addition, a setting time as along as 60 minutes or more is required.
However, magnet materials in general undergo demagnetization when subjected to heat. The anisotropic Nd magnet material, in particularly, is extremely susceptible heat; the magnetic characteristic (BH)max decreases by about 15% when heated at, e.g., 150° C. for 30 minutes.
As for compression molding effected in a magnetic field, a magnetic force is increased by improving density and by enhancing orientation with the magnetic field. However, a problem with the epoxy compound is that density cannot be increased without resorting to high pressure. More specifically, to achieve 13 mGOe, density of 6.1 g/cm3 and therefore pressure of 7.0 ton/cm2 is required. Taking account of the demagnetization by 15% mentioned above, density of 6.55 g/cm3 and therefore pressure of 11.1 ton/cm2 is necessary.
For example, assuming a 3 mm wide, 2.5 mm high, 30.4 cm long rectangular magnet, a pressing area and a pressure required of a horizontal magnetic field system, which applies a magnetic field in a direction perpendicular to a pressing direction, are 7.6 cm2(=0.25×30.4) and 84.42 tons, respectively. As a result, a press belonging to a 100-ton class must be used.
In the case of magnetic Field type of compression molding, after a mold has been located between a pair of electromagnets, a magnetic field is applied between the electromagnets for thereby orienting a magnet. At this instant, the magnetic field is dependent on a gap between the electromagnets, more precisely between iron cores thereof; the narrower the gap, the stronger the magnetic force. The gap between the upper and lower punches of a conventional magnet molding section is 10 mm, so that the pressure of the mold cannot be increased. Consequently, high-pressure damages the mold. It is therefore desirable to use pressure low enough to protect the mold from damage.